This invention relates to a method and apparatus for continuously annealing steel sheet, and more particularly to a method and compact apparatus for continuously annealing steel sheet that permits implementing overaging over a long period of time.
In continuous annealing of steel sheet in strip form (hereinafter called steel strip), as is wellknown, steel strip as cold-rolled is heated to a temperature of approximately 700.degree. to 850.degree. C., soaked for approximately 1 minute for recrystallization, and then cooled rapidly to approximately 400.degree. C. to allow the carbon in the steel to become supersaturated. Then, the steel is subjected to overaging for 2 to 3 minutes in a furnace maintained at approximately 400.degree. C. to cause the solute carbon to precipitate that is detrimental to the workability of the product strip.
Conventional annealing with this heat cycle is adequate for the manufacture of steel strip of ordinary working quality, but not for deep-drawing quality steel that is required to have a particularly high degree of workability. This is because the overaging of 2 to 3 minutes is not enough to cause the solute carbon to precipitate adequately. The result is a lowering in workability that appears as a rise in yield point, a decrease in elongation and the development of yield point elongation through the process of aging deterioration after manufacturing. A solution to this problem has been to reduce carbon content to a minimum in the steelmaking process, with a minute amount of residual carbon fixed as a compound by adding titanium or other appropriate element. The high-workability steel thus produced is inevitably costly though it eliminates the need for overaging treatment. Attaining high workability with ordinary inexpensive steel (containing 0.03 to 0.05 percent carbon) calls for overaging treatment of long duration, which has simply been impracticable with the conventional vertical annealing furnace equipped with a large-diameter hearth roll because the equipment size would become tremendously large.
Generally aging deterioration is evaluated in terms of aging index. It is said that steel is suited for deep drawing if its aging index is approximately 3 kg/mm.sup.2 or under. To attain an aging index of not higher than 3 kg/mm.sup.2 with the inexpensive steel just mentioned, the steel must be overaged for a period of 20 to 30 minutes. This overaging time is more than 10 times longer than that in the conventional annealing heat cycle. In order to carry out this long overaging on a conventional vertical furnace, the overaging section alone must have a length of 300 m to 500 m, which is simply impracticable.
Japanese Patent Public Disclosure No. 100635 of 1983 discloses a continuous annealing apparatus that permits implementing overaging with a compact furnace. As shown in FIG. 1, this apparatus is designed to hold a large quantity of steel strip 1 in a limited space by winding the strip into a loosely coiled form. In order to keep the outside and inside diameters of a loose coil 3 from changing as the strip 1 travels forward, both the outside and inside of the loose coil 3 are forcibly restrained by guide rolls 5 and 6. Since the peripheral speed of the strip is kept constant on this type of apparatus, however, the angular speed of the strip 1 with respect to the center of the coil 3 increases toward the inside. Therefore, slip between wraps of the strip 1 is unavoidable. When the number of wraps increases, in addition, the cumulative frictional force between the individual wraps grows too large for the guide rolls 5 and 6 to maintain the outside and inside diameters of the coil within the desired limits.
With a conventional vertical continuous annealing furnace using hearth rolls, strip is bent to the radius of curvature of a hearth roll when the strip is turned into a different direction, whereupon stress-aging is likely to occur. Therefore, it has been necessary to use hearth rolls with a considerably large diameter, such as one meter or more, or make some other provisions to inhibit or avoid such stress-aging.